The present invention relates to plastic film, such as biaxially oriented plastic film, and to methods of making such film so that the film exhibits a high degree of straightness.
Biaxially oriented plastic film, such as PET, nylon, polypropylene, PVC, etc., is used in numerous applications which require that the film exhibit a high degree of inherent straightness. For example, plotters such as electrographic plotters, pen plotters, laser plotters, etc., used in the design of aircraft and automobiles function to plot lines on electrographic film. The film is pulled through the plotter as lines are inscribed thereon. Upon being removed from the plotter, it is essential that the film remain straight to insure that the lengths of the lines do not change. It will be appreciated that if the film were to bend within its own plane, the lines formed on the film would also bend, whereby the linear lengths of the lines would be reduced. A shortening of even ten mils per one hundred inches of line length may be unacceptable in certain situations. As an example, such a situation arises in the making of drawings for aircraft parts requiring tight manufacturing tolerances.
It will also be appreciated that if two lines of equal length were formed, one at each side of the web, the shortening of the two lines that would occur were the film to bend, would be unequal for each line and would result in unequal line lengths. A difference in the length of the lines of even ten mils per one hundred inches of line length may be unacceptable in certain situations, as mentioned above.
Those restrictions are of importance, because of the particular characteristics imparted to the film by the manufacturing process. For example, film formed of biaxially oriented PET is typically formed by a technique in which PET material is stretched biaxially, i.e., stretched in the machine direction as well as the cross direction, in the process of being formed into a sheet 10 of film of given width W (see FIG. 1). The film is then cut lengthwise into strips 12 of narrower width W' and processed into a final product, such as electrographic film.
However, due to the internal stress profiles imparted to the film sheet 10 by the biaxial stretching, each individual strip will inherently tend to bend within its own plane, as shown in a somewhat exaggerated manner in FIG. 2. (It is possible that a strip cut from the center of the sheet would not exhibit such a bending tendency if the center of stress of the sheet happened to coincide exactly with the longitudinal center of the strip cut therefrom and if the stress profile were symmetric about the longitudinal center of the web.) If a strip exhibiting such a bending tendency were passed through an electrographic plotter, the strip would be temporarily straightened by the tension applied by the feed mechanism, so that straight lines would be inscribed thereon by the plotter. However, once removed from the plotter, the strip would reassume its curved configuration, whereby the lines would become unacceptably shortened and two lines of originally equal length would become unacceptably unequal in length.
In order to avoid the above-described problem, it has been the practice to attempt to cut a strip 12 from the center of the sheet 10 such that the center of stress of the sheet coincides with the longitudinal center of the strip. This is very difficult and time consuming to accomplish, however, because the center of stress of the sheet does not necessarily lie along the geometrical center of the sheet, nor is the stress profile necessarily symmetric about the geometrical center of the sheet.
Another possible way of dealing with the bending problem would be to trim the curvature from the sides of a curved strip 12, i.e., by cutting the strip of FIG. 2 along trim lines 14. However, it will be realized that, due to the curvature of the strip, the longer the strip, the shorter will be the width of the resulting trimmed strip. Hence, this technique would be viable only to make strips of limited length.
Furthermore, the center-cut technique and the trimming technique described above would each result in a considerable waste of film since the outer portions of the film sheet, or the trimmed portions of each film strip, would be discarded.
Therefore, it would be desirable to provide a straight strip of biaxially oriented plastic film of any desired length without resulting in an appreciable wastage of film.